Heat exchanger



FIG. 1 is a perspective view of a heat exchanger, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is an end elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is an elevational view of the end opposite that shown in FIG. 3;

FIG. 6 is a bottom plan view thereof; and

FIG. 7 is a top plan view thereof. 

The ornamental design for a heat exchanger, as shown. 